Many different types of wire are utilized to perform a wide variety of tasks within the bore of a well; which are collectively referred to as wireline. Common terms for the various types of wireline include slickline, braided line, electric line, logging cable, or simply cable. Wireline is typically spooled onto a winch and fed into a well over sheaves that center the wire vertically over the well. Wireline tool strings are sometimes deployed downhole on wireline; where the tool strings include various tools connected together and which occasionally include weights. The weight of the tool string pulls the wire from the winch and thus gravity feeds into the well. Because the well bore is often pressurized, a pack off assembly is typically installed at the wellbore opening to create a seal around the wire and contain the wellbore pressure. When deploying wire into a pressurized well bore, the tool string must weigh enough so the force of gravity on the tool string exceeds the resistive force generated by well pressure acting on the area of the wire.
Many wellbore operations require an impact to achieve the desired result. Whenever impact is required, tools commonly referred to as jars are installed in the tool string. Jars work much the same as a slide hammer whereby some components are arranged to freely travel a certain distance (stroke), and then impact a shoulder; which instantly halts the upward or downward motion of the tool strings mass thereby creating an impact force. Manipulating jars to create an impact usually requires rapidly manipulating the winch when raising or lowering the tool string, thereby rapidly hoisting or dropping the mass of the tool string. One of the aforementioned sheaves normally contains or is attached to a load cell device that measures the strain on the wire displaying the weight of the wire and tool string. The load cell is usually constantly monitored as the wire is lowered or hoisted within the well bore. Contact with an obstruction that impedes or stops the descent of the tool string is detected by observing a rapid loss of weight on the load cell display. Conversely, when hoisting the wire, the load cell will display a rapid increase in weight if the ascent of the tool string is impeded or halted. While manipulating the winch to deliver an impact, the load cell is monitored for an indication that the jars have reached the end of their free stroke. At the end of the free stroke, winch rotation must stop instantly to prevent damage to the wireline.
Rapidly manipulating the winch to create impact requires skill, and is sometimes problematic due to the wire reciprocating through the sheaves at a high rate of speed. Continued reciprocation, particularly at high speed, can cause wire breakage or fatigue. Lowering the winch rapidly may cause excessive slack in the wire and result in a loosely wound winch or cause the wire to jump out of a sheave. As the wire is lowered by the winch, the wire is stripped through the pack off and drags on the walls of the well bore which tends to impede the descent of the tool string thus dampening downward impact. Furthermore, because wireline cannot effectively push or shove, downward impact is dependent upon the skill of the winch operator, gravity, mass of the tool string, and speed of the winch.
Slow winch rotation to manipulate jars in an upward direction is achieved by utilizing special types of jars. These upward acting jars are commonly referred to as hydraulic jars, oil jars, spring jars, detent jars, or mechanical jars. Briefly explained, these upward jars freely stroke to the closed or downward position but oppose tension until a pre-determined load or time interval is reached. Once the pre-determined load or time interval is reached, the upward acting jar will release and allow the energy stored in the tensioned wire to rapidly move the mass of the tool string upward creating an impact. Because of the energy stored in the tensioned wire, very high upward impact forces can be achieved with these types of jars.